In recent years, along with the increase in the operating speed of personal computers, the spread of network infrastructure, the markedly increased capacity of data storage, and the decrease in price, a further opportunity is increasing in which information such as text and images which have traditionally been supplied in the form of paper printed matter is now acquired and read as convenient electronic information.
Employed as such electronic information reading means are conventional liquid crystal displays as well as CRTs, and in recent years, emissive displays such as organic EL displays are drawn attention. Particularly, when electronic information is in the form of text, it is necessary for an operator to watch the browsing means for a relatively long period of time, which is not a human-conscious operation. Generally, it is known that emissive displays exhibit the following disadvantages: (i) screen flickering results in eye fatigue; (ii) display devices are not portable; (iii) reading posture is limited; (iv) operators tend to gaze at a still screen, and (v) reading over an extended period of time results in an increase of electric power consumption.
Known as display means to overcome such disadvantages are reflective displays (having a memory function) which do not consume power for maintaining images since these displays use external light. However, due to the following reasons, it is difficult to mention that sufficient performance is achieved.
Namely, displays employing polarized plates such as reflective liquid crystals exhibit reflectance as low as approximately 40 percent, resulting in having a problem in displaying white. Further, the production methods employed to prepare constituting members are not simple. Polymer dispersion type liquid crystal displays require relatively high voltage, in addition, since only the difference in the refractive index among organic materials is utilized, the resulting images do not exhibit sufficient contrast. Polymer network type liquid crystal displays result in problems in which a high voltage is required and in order to enhance memory functions, complicated TFE circuits are required. Display elements based on an electrophoretic method necessitate voltage as high as at least 10 V, and durability problems may occur due to coagulation of electrophoretic particles. Electrochromic display elements can be driven at as low as 3 V, however, color qualities of black and other colors (yellow, magenta, cyan, blue, green, and red) are not sufficient, and in addition, in order to keep the memory function of the display, complicated display constitution may be needed, for example, a constituting layer prepared by an evaporation technique.
Known as a display system which overcomes many of the above mentioned problems is an electrodeposition (hereinafter referred to as ED) display utilizing dissolution and deposition of metals or metal salts. The ED display exhibits advantages such that it is possible to drive the ED system at a voltage below 3 V, the cell structure is simple, the contrast between black-and white, as well as the quality of black are excellent. Various methods on ED displays have been disclosed (refer, for example, to Patent Documents 1–3).
However, in the conventional techniques, memory function of the display element (stability of the blackened image) has not been fully attained.                (Patent Document 1) U.S. Pat. No. 4,240,716        (Patent Document 2) Japanese Patent Publication No. 3428603        (Patent Document 3) Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 2003-241227        